Airplay in Sync on iPhone but Not in Mountain Lion
I'm on a late-2011 MacBook Pro with a Sonos wireless speaker system connected to an Airport Express, and I am having the same issue with Airplay being out of sync in iTunes. It's mostly noticeable on video. I expect it to be out of sync with the OS mirroring, but I would think that iTunes would be able to sync the audio and video via AirPlay like the iPhone Videos app does.